This invention relates to an optical recording medium in which a data area is constituted by a pit string made up of pits formed along the track center scanned by a laser beam and lands (mirror surface sections) and in which the pit string in the data area is read with pre-set clocks so as to be reproduced as information signals, a method for reproducing data recorded on the optical recording medium, and a data recording device or laser cutting device employed for producing the optical recording medium.
An optical recording medium, such as one rotated at CAV (constant angular velocity), referred to herein as an optical disc, has a recording format, such as a recording format for a data area as shown in FIG. 21, in which pits P are formed along a track center Tc, with a pit width d of 0.5 .mu.m and a pit length per clock of 0.86 .mu.m and at a track pitch Tp of 1.6 .mu.m along the disc radius, and are aligned in their positions along the track direction. The portions devoid of the pits are lands left as mirror surface regions.
For reproducing information signals from the optical disc having the above-mentioned recording format, the disc is rotated at CAV and a playback laser beam is radiated on the track center Tc for relative scanning with a laser spot BS.
The laser beam reflected from the optical disc is caused to fall on a light receiving element and converted into detected signals as electrical signals having a signal level corresponding to the reflected light volume. The detected signals are further demodulated by a signal demodulating circuit for producing playback information data.
The reflected light falling on the light receiving element after reflection by the mirror surface region is the light which has undergone substantially total reflection by the mirror surface. Thus the reflected light volume is abundant so that a detection signal having a high signal level is outputted from the light receiving element. On the other hand, the light volume of the reflected light modulated by the pit is lesser than that of the reflected light from the mirror surface region, so that a detection signal having a low signal level is outputted from the light receiving element.
In a downstream signal processor, detection signals outputted in series by the light receiving element are sampled with pre-set clock signals and thereby converted to bi-level data having a pulse amplitude corresponding to the signal level. The bi-level data is processed for decoding error-correction codes, such as parity codes or interleaving, for producing playback information data.
Since data corresponding to the pit P is a logical "1" and data corresponding to the mirror surface region is a logical "0", data having a long concatenation of "1"s or "0"s suffers from increased deviation in the dc balance. That is, the digital sum value (DSV) is offset to the (-) side or to the (+) side, thus producing an unstable state of the servo control system.
In addition, such data recording has a drawback that the data length is substantially increased, which is not meritorious in increasing the data recording density.
Another known recording method is shown in FIG. 22 in which recording is made so that the boundary between the pit P and the mirror surface region M is logically "1" and the pit P as well as the mirror surface region M other than the boundary is logically "0". Data reproduction is made in a similar manner. Such recording method is meritorious for increasing data recording density since it is unnecessary to increase the data length in distinction from the firstly stated system.
Consequently, the conventional practice has been to record pits on the optical disc after 8-bit data is converted to 14-bit data in accordance with the eight-to-fourteen modulation (EFM). Playback information data are produced after decoding EFM codes in addition to decoding of the error correction codes, as stated hereinabove.
However, the EFM system is not meritorious for high-density recording because it is 14-bit data converted from 8-bit data that is to be recorded. Although it is desirable to employ a direct data recording system for high density recording, the above-mentioned problem caused by increased dc balance offset is raised.
Besides, with the conventional optical disc, since the recording data are implemented by a bit string pattern consisting of mirror surface sections M and pits P formed on the track center Tc, the number of pits P and a range in which the pits P are formed differ from track to track depending on recording data. That is, the proportion between the number or size of the pits P and the number or size of the lands M in a data area per track is not equal and differs from track to track.
This leads to such a situation in the optical disc fabrication that, when a recording pattern on a stamper (a array pattern consisting of pits P and lands M is to be transcribed onto a resin substrate by an injection molding method using the stamper, the flow rate of the molten resin into cavities is not uniform due to the differential density of the protrusions and recesses on the stamper resulting in fluctuations in the state of adhesion of the molten resin to the stamper. The result is that the contour of the pits P of the completed optical disc is locally deviated from the prescribed shape, while molding defects such as interruptions in the mirror surface sections M are produced.
Such molding defects are produced in particular in servo areas of the optical disc constructed in accordance with the sampled servo system. That is, the servo area is usually separated from the data area by a mirror area constituted solely by mirror surface sections M. Thus the mirror area is continuous along the radius of the optical disc. The result is that molten resin flows quickly through the mirror area towards the outer periphery of the cavity, so that a so-called ghost, that is broken edges of servo pits caused by the radially continuous mirror area, tends to be produced.
In addition, with the above-described sampled servo type optical disc, the following problem arises during tracking servo control during reproduction.
That is, the tracking servo control in the conventional sampled servo system is carried out using a pair of wobbled pits Pa and Pb pre-formed with a shift of one-fourth of a track pitch in opposite directions from the track center Tc, as shown in FIG. 23.
Specifically, the amount of reflected light when the laser beam spot BS traverses the wobbled pits Pa and Pb is sampled, and a tracking error signal is found based upon the difference between these signals. The spot BS is moved radially of the optical disc until the signal level, for example, of the tracking error signal becomes equal to zero, by way of performing tracking servo control.
On the other hand, the so-called track jump, which is the movement of the spot to a neighboring or other track, is performed by opening the tracking servo control loop, moving the spot to near a target track and subsequently closing the tracking servo control loop for capturing the spot BS to the target track.
During such track jump, that is when the laser beam spot BS scans the track obliquely, the tracking error signal in the sampled servo system is a sine wave signal, as shown in FIG. 24, such that it is not unequivocally set with respect to a displacement x of the beam spot BS from the track center.
It is when the spot BS is within a range 201, shown by hatched lines, that the tracking error signal is determined unequivocally with respect to the displacement x. That is, it is when the spot BS is within the range 201 that the spot BS can be captured without fail with respect to the track center Tc.
On the other hand, if the displacement x is larger and is outside the range 201, that is within the range 202, tracking servo control becomes unstable. Such unstable state tends to be produced when the laser beam spot BS is moved with an elevated speed along the radius of the optical disc as during the track jump.
There is produced an error in the distance the beam spot BS is moved during track jump with the tracking servo loop being turned off. If the tracking servo control loop is turned on outside the range 201, such as within the range 202, the possibility is high that the beam spot will be captured to a track other than the target track. In such case, a track jump needs to be performed a second time. Thus the conventional tracking servo has a drawback that the track jump cannot be preformed stably.